We have completed several studies on pair formation and maintenance and the effects of the birth of infants on behavior of other group members. In pygmy marmosets we have found high levels of sociosexual behavior in newly formed pairs with increased levels of sociosexual behavior during the female's periovulatory behavior (Converse et al., 1995). This complements our previous results showing that cotton-top tamarins also communicated their ovulatory state to mates. After the birth of infants in pygmy marmosets, there was no change in the levels of sociosexual behavior from the levels of initial pairing except that peri-ovulatory specific activities declined. Parents whose infants died showed higher levels of sociosexual behavior than did parents whose infants survived, although there was no difference in the timing of the postpartum conception (Carlson et al., in press). We have also analyzed five years of infant care data from wild cotton-top tamarins in Colombia and find many parallels to our captive studies (Savage et al., in press). Maternal caretaking is constant at about 25% of the total time. The amount of time adult males took care of infants was reduced as group size increased. Males appeared to time share infant care-taking and vigilance activities. The role of vigilance has probably been underestimated in captive studies. First-time mothers carried infants 90% of the time in the first two weeks compared with 25% carrying seen in experienced mothers. There was an effect of group size on infant survival with 100% survival occurring only with groups of 5 or more adults. We have completed an analysis of the 18 years of breeding records in our colony and to our surprise, we found a similar effect of group size on survival. Infant survival of 100% occurred only with group sizes of four or greater. First time parents had a very low infant survival rate, but did significantly better with their second parity. We also found a significant sex ratio by parity interaction with significantly more males being produced in the first pregnancy and significantly more females being produced after the fifth pregnancy (Greene and Snowdon, in preparation). The birth of new infants has a dramatic effect on the behavior of juveniles in the group. Juveniles showed a high level of curiosity and interest in new infants on the day of birth, but were recipients of high levels of aggression from those carrying infants. After the first day, juvenile interest in infants declined, but by the fifth week after birth, juveniles were involved in infant caretaking (Achenbach, in preparation). We are analyzing infant care-taking patterns in pygmy marmosets and cotton-top tamarins to determine if there are species differences parallel to those found with common marmosets, and how the number of infants and size of infants affects the distribution of caretaking behavior. All of these studies will lead to a better understanding of how monogamous pairs are formed and maintained, and how several group members cooperate in the successful rearing of infants. Since marmosets and tamarins live in family groups similar to those of most humans, we expect these results to provide insights on improving parental care and pair relationships in humans.